1. Field of the Invention
The present invention relates to a light-emitting device, a method for making the same, and a nitride semiconductor substrate. More specifically, the present invention relates to a light-emitting device formed from a nitride semiconductor, a method for making the same, and a nitride semiconductor substrate. In the present invention, a “light-emitting device” may refer to a semiconductor element formed essentially from a nitride semiconductor substrate and a semiconductor layer laminated thereon, or may refer solely to a semiconductor chip, or may refer solely to a device sealed in resin with a semiconductor chip mounted in a mounting part. Furthermore, the term may also be used with both meanings at the same time. Also, a semiconductor chip may be referred to solely as a “chip”. Also, in a chip, a substrate and an epitaxial layer formed thereon may be referred to simply as a “substrate”.
2. Description of the Background Art
Currently, white light-emitting diodes (LED) are frequently used for illumination and compact electronic devices such as portable information terminals. However, there is the potential that they may be used for illumination in large spaces or over large areas in the future. To make it possible to use LEDs for illumination of large spaces and large areas, the light output of LEDs must be increased.
One method for increasing the light output of an LED is to efficiently output the light generated inside the LED to the outside, i.e., to improve the light extraction efficiency. Examples of conventional technologies for improving light extraction efficiency include: a light-emitting device with a substrate formed from sapphire or the like wherein a light-reflecting surface with a convexo-concave shape is formed using blasting on a back surface opposite from a surface on which a nitride semiconductor layer is formed (e.g., see Japanese Laid-Open Patent Publication Number 2004-56088); a light-emitting device wherein a curved shape that, combined with groove formation and abrasion, can act like a lens is formed on the back surface described above of a substrate formed from sapphire or the like; and a gallium nitride (GaN) device laminated on a sapphire substrate, an electrode being formed on the n-GaN layer and a hemispherical projection with a diameter of 0.5-1 micron formed on the n-GaN layer surface by etching the n-GaN layer surface (e.g., the non-patent document: D. W. Kim, and three others, “Highly efficient vertical laser-liftoff GaN-based light-emitting diodes formed by optimization of the cathode structure”, Applied Physics Letters 86, 052108 (2005)).
In the light-emitting device disclosed in Japanese Laid-Open Patent Publication Number 2004-56088, blasting is performed to form a convexo-concave shape, but accurately controlling the shape of the concave/convex shape when this method is used is difficult. Also, in Japanese Laid-Open Patent Publication Number 2004-56088, dicer and laser processing are also given as examples for methods forming a reflective surface, but forming shapes such as hemispherical surfaces is difficult with these methods.
Also, in the specification of Japanese Patent Number 3362836, a curved-surface shape is formed by forming a groove using a dicer or laser and then applying abrasion, but since this method combines a dicer step and an abrasion step, the number of production steps is increased compared to simply using a dicer by itself. As a result, the production cost of the light-emitting device is increased.
In the non-patent document described above, a hemispherical projection is formed by etching the surface of a GaN layer (n-GaN layer) laminated on a sapphire substrate so that the diameter of the formed projection is limited by the thickness of the laminated GaN layer. More specifically, the GaN layer must be laminated with adequate thickness in order to form a projection with a larger diameter, but the laminating step for a thicker GaN layer requires more time, resulting in increased production costs for the light-emitting device.
The object of the present invention is to overcome these problems and to provide a light-emitting device, method for making the same, and a nitride semiconductor substrate that can be used for making the same wherein light extraction efficiency can be improved without increasing production costs.